In this July 6, 2010, photo, Lance Armstrong waits for the start of the third stage of the Tour de France cycling race in Wanze, Belgium. US prosecutors have dropped a two-year doping investigation of Armstrong.

Christophe Ena/AP

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ATLANTA — The decision by US prosecutors to drop a two-year doping investigation of legendary Tour de France champ Lance Armstrong could mark a stalemate in the debate over how to deal with the use of anabolic steroids in sport.

The use of illegal performance-enhancing drugs by athletes striving to win at all costs has rocked baseball, track and field, and cycling in the last decade. Congress has addressed the issue in recent years and medals and trophies have been returned by athletes like cyclist Floyd Landis and track and field star Marion Jones, even as major US sports leagues have instituted strict anti-doping policies in response to public restiveness about fairness.

But the decision by US Attorney Andre Birotte on Friday to close the Armstrong investigation comes after a number of high-profile prosecutions managed to raise awareness about the seriousness of doping, but failed to produce significant legal victories against the defendants.

In a 2011 federal trial, retired home run king Barry Bonds escaped serious criminal charges after becoming one of 86 baseball players in the 2007 Mitchell Report linked to doping. Last year, the judge in the Roger Clemens perjury trial quickly declared a mistrial after a prosecutor introduced disallowed evidence to jurors. Clemens is slated for a retrial in April.

In both those cases, the government wasn't trying to prove the sports stars took illegal steroids, but that they lied to investigators – and, in Clemens' case, to Congress – during the investigation.

Armstrong, meanwhile, was being investigated for possible crimes ranging from defrauding the government, drug trafficking, and conspiring with other cyclists to distribute the substances. The main thrust was to determine whether money from the US Postal Service, Armstrong's principal sponsor during his first four Tour de France wins, was used to buy the drugs.

Mr. Birotte, assisted by the same investigator – Jeff Novitzky of the Food and Drug Administration – who helped prepare the Bonds and Clemens cases, did not expressly state why the government was closing the Armstrong investigation.

“The United States Attorney determined that a public announcement concerning the closing of the investigation was warranted by numerous reports about the investigation in media outlets around the world,” Birotte said in a statement.

To be sure, critics say the decision is shocking given gripping testimony from close Armstrong associates, including allegations detailed by several former teammates in a “60 Minutes” report last May, that pointed to rampant doping by several teams associated with Armstrong, and testimony from fellow riders who claim they saw Armstrong use banned substances.

But the problem for prosecutors is judging the strength of testimony with the mood of the potential jury pool. While finding Bonds guilty on one charge of obstructing justice, earning him 30 days of house arrest, the judge declared a mistrial on three charges, and the jury couldn't agree to convict on several more serious allegations that could've given Bonds jail time.

In the public eye since 1928, doping has both changed the nature of athletics and forced changes in how sporting authorities, including the NFL, MLB and the NHL, as well as the Olympic Committee, address the corruption of sporting events.

Athletes still risk their careers and reputations by cheating, but the closing of the Armstrong investigation also suggests that the public is hesitant to punish an athlete with jail time for trying to gain a competitive advantage. Procuring hard proof has also been difficult, and juries are liable to consider allegations against popular athletes as hearsay coming from people who may have their own motivations for testifying against sports stars.

Given all that, the closing of a case against Armstrong that seemed to confirm fears even among many of the cyclist's legion of fans hints at a careful legal calculation likely to mark a significant moment in the doping debate.

“Unlike the U.S. Attorney, USADA’s job is to protect clean sport rather than enforce specific criminal laws,” Mr. Tygart said in a statement.

Armstrong, meanwhile, has always forcefully denied doping, and he recently hired powerful attorneys and a public relations firm while continuing his work with his Live Strong cancer awareness organization. In the end, one single fact may have stood above the rest: Armstrong never once tested positive for steroids.

As far as at least the courts are concerned, Armstrong won his seven Tours de France fair and square.